Electric heating elements are used in different sectors to generate heat; they require high voltages in order to generate a sufficiently high temperature. These high voltages, however, can constitute safety risks, particularly when used to heat media or when in contact with the human body. Moreover, because of the materials used in them, most traditional heating elements are suitable only for low temperatures, particularly in long-term operation. Other proposals of the prior art require a complex constitution of the heating element und hence limit possible applications of the heating element.
It is one object of the present invention to provide a heating element with which a high output per unit area and thus high temperatures can be generated even in long-term operation while low voltages prevail in the heating element. In addition, the heating element should be versatile in its applications and simple to provide with contact terminals. This is especially important for hollow structures to be heated such as pipes, transportation devices, e.g. heatable tanks or other containers, or heat roller shells.
Pipes are extensively employed, for instance, to conduct fluid media. When such pipes for instance are laid underground or as open-air piping in cold regions, the risk exists that the medium present in the pipe solidifies because of the low temperatures, and the pipe clogs.
Media such as gases or liquids often are transported in heatable tanks mounted on railway cars or on trucks. At low ambient temperatures, the medium in the tank can freeze and thus may even damage the tank. The installation of heating elements in such cars is highly demanding with respect to the heating element as well as to the heat transfer that can occur between the heating element and the car. Dangerous substances sometimes are transported in such tanks. It is important then that the heating element will not lead to any local temperature increase. But also a failure of the heating element, for instance as a result of its detachment from the tank, must be avoided in order to prevent freezing of the medium.
It is therefore a further object of the present invention to provide a heating element for a transportation device in which during transport a medium can be kept at a predetermined temperature, without creating safety risks such as freezing, an explosion or a fire.
Heat rollers which can be heated to a certain temperature, particularly for use in copying or foil-coating machines, are required in many areas of heating technology. Up to now such heat rollers have been produced with heating elements having resistance wires embedded in an insulating mass. Another operating mode of heat rollers, for instance in copiers, is the installation of a halogen emitter in the roller. Both of these versions have the disadvantage of being either very expensive in their manufacture or exhibiting a poor efficiency of heat transfer.
It is therefore a still further object of this invention to provide a heating element for a heat roller of simple design that can be operated with low voltage and at the same time has a high heat transfer efficiency. The heat roller should further be versatile in its applications.
The invention is based on the realization that all these objects can be reached by a heating element in which the heating current flows in an optimum way through a suitable resistive mass, the heating element being of flat shape and guaranteeing a heat transfer that is uniform across the area to be heated.
According to the invention, the objects are reached by a flat heating element comprising a thin resistance layer which contains an electro-conductive polymer and at least two flat electrodes arranged on one side of the resistance layer at a distance from each other, wherein the polymer has an intrinsic electric conductivity caused by a content of at least one metal or semimetal atom dopant.
These polymers which, according to the invention, are used in the resistance layer have a constitution such that the current flows along the polymer molecules. Owing to the polymer structure, the, heating current is conducted through the resistance layer along the polymers. Because of the electric resistance of the polymers, heat is generated which can be transferred to an object to be heated. Here the heating current cannot follow the shortest pathway between the two electrodes but follows the structure of the polymer arrangement. Thus, the length of the current path is predetermined by the polymers, so that even in the instance of small layer thicknesses, relatively high voltages can be applied without causing a voltage breakdown. Even in the instance of high currents such as making currents, one must not be afraid of a burn-out. Moreover, the distribution of the current in the first electrode and its subsequent conduction along the polymer structure in the resistance layer leads to a homogeneous temperature distribution within the resistance layer. This distribution arises immediately after applying voltage to the electrodes.
Because of the polymers employed according to the invention, the heating element, the pipe, the transportation device and the roller shell to be heated can be operated even at high voltages, for instance line voltage. As the attainable heating power increases with the square of operating voltage, the resistance-heating element, the pipe, the transportation device and the roller according to the invention can yield high heating power and hence high temperatures. According to the invention, the current density is minimized because a relatively long current path is provided along the electro-conductive polymers or because at least two zones are created which are electrically in series and contain the intrinsically electro-conductive polymer used according to the invention.
Moreover, the electro-conductive polymers used according to the invention exhibit long-term stability. This stability is explained above all by the fact that the polymers are ductile, so that a rupture of the polymer chains and thus interruption of the current path will not occur when the temperature is raised. The polymer chains are unharmed even after repeated temperature fluctuation. In conventional heating elements, to the contrary, where conductivity is created, for instance, by carbon black skeletons, such a thermal expansion would lead to interruption of the current path and hence to overheating. This would lead to a strong oxidation and to bum-out of the resistance layer.
Similar considerations apply to konwn intrinsically electro-conductive polymers containing chemical compounds and/or ions which sharply reduce the long-term stability of the resistance layer subject to electric currents. It was revealed that polymers which contain a higher percentage of ions have a low aging resistance when subject to electric currents, since electrolysis reactions lead to spontaneous destruction of the resistance layer.
The intrinsically electro-conductive polymer used according to the invention is not subject to such aging phenomena; they resist aging even in reactive environments such as air, let alone oxygen. Moreover, current conduction through the resistive mass is of the electronic conduction type. Hence even an autodestruction of the resistance layer by electrolysis reactions caused by electric currents will not occur in the heating element according to the invention in which time-dependent drops in heating power per unit area are very small and approximately zero, even at temperatures as high as e.g. 500xc2x0 C. and at heating powers per unit area as high as e.g. 50 kW/m2.
Due to the use of intrinsically electro-conductive polymers, the resistance layer as a whole which is used according to the invention presents a homogeneous structure that permits a heating that is uniform across the entire layer.
According to the invention, contact to the heating element is provided by two electrodes which preferably consist of a material of high electric conductivity and are arranged on one side of the resistance layer. This type of contact arrangement makes it possible to use the mode of operation of the inventive heating element in a particularly advantageous way. The current applied first spreads within the first electrode, then crosses the thickness of the resistance layer along the polymer structure, and finally is conducted to the second contacted electrode. Therefore, the current path is additionally extended over that present in a structure where the resistance layer is sandwiched between the two electrodes. Due to this flow of the current the thickness of the resistance layer can be kept small.
The heating element with which the hollow structure, i.e. the pipe, the transportation device and/or the roller shell are to be heated according to the invention, has the further advantage of being versatile in its applications. The electrodes are provided with contacts on one side of the resistance layer. The opposite side of the resistance layer therefore is free of contact terminals, and hence can be of flat shape. Such a flat surface permits a direct application of the heating element to the structure to be heated. An ideal heat transfer becomes possible since the contact area between the heating element and the structure to be heated is not disrupted by contact terminals if the surface of said structure consists of a material having a high electric conductivity.
If this is not the case, in another preferred embodiment of the invention, a flat floating electrode is arranged on the side of the resistance layer opposite to the two flat electrodes. In the spirit of the invention, an electrode is called floating when it is not connected to the source of current. It can have an insulation preventing electric contact with a source of current.
This floating electrode supports the flow of current through the resistance layer. In this embodiment the current spreads within the first electrode, crosses the thickness of the resistance layer to reach the floating electrode on the opposite side, is conducted further within this electrode, and finally flows through the thickness of the resistance layer to the other electrode that is arranged on the same side of the resistance layer as the first electrode.
In this embodiment of the heating element the current flows through the thickness of the resistance layer, essentially in a direction normal to its surface. Essentially two zones develop within the resistance layer. Within the first zone, the current flows essentially vertically from the first contacted electrode to the floating electrode, while within the second zone, it flows essentially vertically from the floating electrode to the second contacted electrode. Thus, a series arrangement of several resistances is attained by this arrangement. This effect implies that the partial voltage prevailing in the individual zones is smaller than the applied voltage. Thus, in this embodiment of the invention the voltage prevailing in the individual zones is half of the applied voltage. Because of the low voltage prevailing in the resistance layer, safety risks can be avoided with the heating element according to the invention, and possible applications thus are manifold and not limited to the before-mentioned hollow structures.
The heating element can then also be used in devices where it comes in immediate contact with a medium to be heated, or must be touched by the persons which operate or use the device. A pipe fitted with the heating element according to the invention can be employed in wet areas or moist ground or find applications where people must touch the pipe. The transportation device according to the invention can thus also be used in applications in which people must touch the container. In the transport of media, the device according to the invention is exposed to atmospheric conditions. Thus, the device can come in contact with water, particularly in rain or snow. However, a safety risk will not arise by this contact because of the extremely low voltage prevailing in the resistance layer of the heating element according to the invention which may be operated with a conventional power source such as a battery. This can readily be mounted on the railroad car or truck. In the latter instance the device according to the invention can even be powered by the truck""s battery, which represents an additional design simplification.
Moreover, the gap provided between the contacted electrodes acts as an additional resistance arranged in parallel. With air as the insulator in this gap, the resistance will be determined by the mutual distance of the electrodes and thus by the surface resistance of the resistance layer. The distance is preferably larger than the thickness of the resistance layer, for instance twice the thickness of the resistance layer.
The electrodes and the floating electrode preferably have a good thermal conductivity. This can exceed 200 W/mxc2x7K, preferably 250 W/mxc2x7K. Local overheating can rapidly be neutralized by this good thermal conductivity in the electrodes. An overheating is thus possible only in the direction of layer thickness, but has no negative effects because of the small layer thickness that can be realized in the heating element according to the invention of which it is a further advantage that even a local temperature increase provoked from outside, e.g. from the body to be heated, can be balanced in an ideal way. Such an increase in temperature can occur for instance in pipes or with containers only partly filled, since in zones that are filled with air, less heat is transferred from the pipe or the container to the air. Such a temperature rise can also be produced from the inside, for instance when an accumulation of heat occurs in the heat roller. For this reason a thermal insulating material can be provided inside the roller.
The electrodes and the floating electrode are preferably made of a material having a high electric conductivity. Thus, the specific electric resistance of the electrodes may be less than 10xe2x88x924 xcexa9xc2x7cm, and preferably less than 10xe2x88x925 xcexa9xc2x7cm. Suitable materials are e.g. aluminium and copper. By selecting such an electrode material it is guaranteed that the current applied is conducted further within the flat electrode, i.e., spreads within it, before passing through the resistance layer. This leads to a uniform flow of the heating current through the resistance layer and thus a uniform and essentially complete heating of the resistance layer. Such a heating element therefore is able to generate and transfer heat in a uniform way. By selecting such an electrode material it is possible in particular to fabricate large heating elements without a need for voltage supply to a number of spots along the length or width of the electrodes. Therefore, power supply lines need not be installed along the surface. According to the invention, such multiple contacts will only be selected for embodiments in which the heating element covers a large area or length, for instance areas larger than 60 cm2, preferably larger than 80 cm2. The limiting size of the heating element above which it becomes meaningful to provide multiple contact points depends not only on the electrode material selected, but also on the place of the contacts. Thus, multiple contact points may not be required even for areas larger than those mentioned above when the electrode is accessible in its surface midpoint and can be provided with a contact there.
The size of the heating element, the length of the pipe and the transportation device as well as the heat-up rate and the temperature generated across the roller surface further depend on the thickness of the selected electrodes that can be operated, with single contacts According to one embodiment, the electrodes andxe2x80x94if presentxe2x80x94the floating electrodes have a thickness of 50 to 150 xcexcm, preferably 75 to 100 xcexcm each. These small layer thicknesses are also advantageous in that the heat produced by the heating element can readily be transferred from them, e.g. from the interlayer to the body to be heated. Moreover, thin electrodes are more flexible, so that a detachment of the electrodes from the resistance layer and thus an interruption of the electrical contact during thermal expansion of the resistance layer will be avoided.
According to the invention, the resistance layer is thin. Its thickness has a lower limit that merely depends on the breakdown voltage, and is preferably 0.1 to 2 mm, preferably about 1 mm. A small layer thickness of the resistance layer offers the advantage of enabling a short heat-up time, rapid heat transfer and high heating power per unit area. However, such a layer thickness is only possible with a heating element according to the invention. On one hand, the current path within the resistance layer is predetermined by the polymers used according to the invention, and can be sufficiently long to prevent voltage breakdown, even when the layer thicknesses are small. On the other hand, the unilateral contact arrangement of the heating element permits subdivision of the resistance layer into zones of lower voltage, which additionally reduces the risk of breakdown.
The advantages of the heating element and the use of heating elements according to the invention are further enhanced when the resistance layer has a positive temperature coefficient (PTC) of its electric resistance. This leads to an effect of automatic regulation with respect to the highest attainable temperature. This effect occurs, since the flow of current through the resistive mass is adjusted as a function of temperature because of the PTC of the resistance layer. The current becomes lower the higher the temperature, until at a particular thermal equilibrium it has become immeasurably small. A local overheating and melting of the resistive mass can therefore be reliably prevented. This effect is of particular importance in the present invention, related to following reasons:
For the heating element according to the invention, a local temperature rise may occur, for instance, when the heating element according to the invention has insufficient contact with a body to be heated, and hence a low heat transfer. If for instance a heatable pipe or container is only partially filled with a liquid medium, heat is more readily withdrawn from the filled region of the pipe or container than from its region in contact with air. A conventional heating element would heat up and perhaps melt because of deficient heat withdrawal. Such a melting is avoided by the effect of automatic regulation in the heating element according to this invention.
Selecting a PTC material for the resistance layer also implies, therefore, that as a result, the entire resistance layer is heated to essentially the same temperature. This enables uniform heat transfer, which can be essential for particular applications of the heating element and use of heating elements, for instance when heat-sensitive media are conveyed through the pipe or transported in the container. For particular applications of the heat roller, non-uniform heat transfer in some spots for instance may cause the foil to be applied by the roller will not adhere to the substrate, since it was not sufficiently and/or uniformly heated.
According to the invention, the resistance layer can be metallized on its surfaces facing the electrodes and/or the floating electrode (if present). By metallization, metal adheres to the surface of the resistance layer and thus improves the flow of current between the electrodes or the floating electrode and the resistance layer. Moreover, in this embodiment, the heat transfer from the resistance layer to the floating electrode and hence to the body or object to be heated (e.g. pipe, container or roller shell) is also improved. The surface can be metallized by spraying of metal.
The intrinsically electro-conductive polymer is preferably produced by doping of a polymer. The doping can be a metal or semimetal doping. In these polymers the defect carrier is chemically bound to the polymer chain and generates a defect. The doping atoms and the matrix molecule form a so-called charge-transfer complex. During doping, electrons from filled bands of the polymer are transferred to the dopant. On account of the electronic holes thus generated, the polymer takes on semiconductor-like electrical properties. In this embodiment, a metal or semimetal atom is incorporated into or attached to the polymer structure by chemical reaction in such a way that free charges are generated which enable the flow of current along the polymer structure. The free charges are present in the form of free electrons or holes. In this way an electronic conductor arises.
Preferably, for its doping, the polymer was mixed with such an amount of dopant that the ratio of atoms of the dopant to the number of polymer molecules is at least 1:1, preferably between 2:1 and 10:1. With this ratio it is achieved that essentially all polymer molecules are doped with at least one atom of the dopant. The conductance of the polymers and hence that of the resistance layer as well as the temperature coefficient of resistance of the resistance layer can be adjusted by selecting the ratio.
The intrinsically electro-conductive polymer used according to the invention can be employed as material for the resistance layer in the heating element according to the invention, even without graphite addition, but according to a further embodiment, the resistance layer may additionally contain graphite particles. These particles can contribute to the conductivity of the complete resistance layer, are preferably not in mutual contact, and in particular do not form a reticular or skeletal structure. The graphite particles are not solidly bound into the polymer structure but are freely mobile. When a graphite particle is in contact with two polymer molecules, the current can jump via the graphite from one chain to the next. The conductivity of the resistance layer can be further raised in this way. On account of their free mobility in the resistance layer, the graphite particles can also move to the surface of this layer and bring about an improvement of its contact with the electrodes or with the floating electrode or interlayer, or with the body to be heated.
The graphite particles are preferably present in an amount of at most 20 vol. %, and particularly in an amount of at most 5 vol. % relative to the total volume of the resistance layer, and have a mean diameter of at most 0.1 xcexcm. With this small amount of graphite and the small diameter, formation of a graphite network which would lead to current conduction through these networks can be avoided. It is thus guaranteed that the current essentially continues to flow by electronic conduction via the polymer molecules, and thus the advantages mentioned above can be attained. In particular, conduction need not be along a graphite network or skeleton where the graphite particles must be in mutual contact, and which is readily destroyed under mechanical and thermal stress, but it rather occurs along the ductile and aging-resistant polymer.
Both electro-conductive polymerizates such as polystyrene, polyvinyl resins, polyacrylic acid derivatives and mixed polymerizates of these, polyamides and their derivatives, polyfluorinated hydrocarbons, polymethyl metacrylates, epoxides, polyurethanes as well as polystyrene or their mixtures can preferably be used to make up the intrinsically electro-conductive polymers. Polyamides additionally exhibit good adhesive properties, which are advantageous for the preparation of the heating element and the use of heating elements according to the invention, since this facilitates applications to the body to be heated. Some polymers, for instance polyacetylenes, are eliminated from uses according to the invention because of their low aging resistance due to reactivity with oxygen.
The length of the polymer molecules used varies within wide ranges, depending on the type and structure of the polymer, but is preferably at least 500 and particularly preferably at least 4000 xc3x85.
In one embodiment, the resistance layer has a support material. This support material on one hand can serve as carrier material for the intrinsically conductive polymer, on the other hand it functions as a spacer, particularly between the electrodes and the floating electrode or interlayer, or the electro-conductive body to be heated. The support material in addition confers some rigidity to the heating element, so that it will be able to resist mechanical stress. Moreover, when using a support material one can precisely adjust the layer thickness of the resistance layer. Glass spheres, glass fibers, rock wool, ceramics such as barium titanate or plastics can serve as support materials. A support material present as a tissue or mat, for instance of glass fibers, can be immersed into, i.e. impregnated by a mass consisting of the intrinsically electro-conductive polymer. The layer thickness then is determined by the thickness of the grid or mat. Methods such as scraping, spreading or known screen-printing methods can also be used to produce said support material.
Preferably, the support material is a flat porous, electrically insulating material. With such a material it can in addition be prevented that the heating current flows through the support material rather than through the polymer structure.
The possibility of producing layers which across their surface deviate from the desired layer thickness with minimum tolerances, for instance 1%, is of particular significance, especially with the small layer thicknesses used according to the invention, since otherwise there is the danger of a direct contact between contacted electrode and floating electrode. Fluctuations in layer thickness across the layer surface can also influence the temperature generated, and lead to a non-uniform temperature distribution.
The support material has the further effect that the current cannot flow along the shortest path between the electrodes and the floating electrode but is deflected or split up at the filler material. Thus, an optimum utilization of the energy supplied is achieved.